The term "fracturing" refers to processes for increasing production from gas and hydrocarbon wells. Fracturing consists of methods which either create and/or increase the size of cracks and crevices within subterranean formations. Fracturing creates larger channels in the formation, thereby allowing greater amounts of gas or hydrocarbons to flow into the well. Fracturing can be performed in many ways, such as through combustive processes, acidic processes or through hydraulic processes.
The term "fluid fracturing" refers to a method of hydraulic fracturing which consists of continuously pumping a fluid at high pressure into a subterranean formation, such as through a well tubular, to reach a wellbottom pressure exerted by the fluid sufficient to force apart cracks and crevices within the subterranean formation. In most instances, the fracturing fluid contains added "proppants." The fluid carries the proppants to the enlarged cracks and crevices within the formation and deposits them there to maintain the fractures after the fluid has been withdrawn. When the fluid is withdrawn, "vented," or "flared," from the formation, gases, such as carbon dioxide, carbon monoxide, hydrogen sulfide, propane, methane, nitrogen and well products, can escape to the atmosphere. "Well products," as used herein, denotes any product, including, but not limited to, gases and other hydrocarbons obtained from subterranean drilling processes. "Wellhead pressure" as used herein, refers to the pressure at which fluids are pumped at the surface into a subterranean formation. Wellhead pressure is to be contrasted with "wellbottom pressure", which, as used herein, is the actual pressure at which fracturing occurs in the formation, which is the sum of the wellhead pumping pressure and the fluid's hydrostatic head pressure.
Fluid fracturing processes are well known. Many types of fluids have been used in fracturing, including liquids, gases, foams and combinations thereof. Foam fracturing fluids consist of both a liquid and gas phase. Foam fracturing fluids are preferred because of their ability to clean-up easily, eliminating or minimizing the need for mechanical withdrawal techniques.
Foam fluids using carbon dioxide liquid in the liquid phase are known. These fluids have the inherent advantage of reducing the amount of water in the liquid phase. When carbon dioxide is added to the liquid phase it reduces the liquid load to be recovered from the formation because the carbon dioxide liquid changes to a gas when pressure on the well is released. Additionally, the evolution of carbon dioxide from the liquid phase to the gas phase "energizes" the liquid phase, causing more complete and rapid cleanup of the liquid load.
U.S. Pat. No. Re. 32,302 by Almond et al., issued Dec. 9, 1986, assigned to Halliburton Company, describes a fluid fracturing process wherein the two-phase fluid utilized contains between 53% and 96% by volume carbon dioxide, with a liquid phase containing liquid carbon dioxide and a foam phase containing gaseous carbon dioxide. The fluid also contains various proppants, foaming agents, gelling agents, surfactants, and other additives, however, it does not contain nitrogen.
The second phase of the fluid disclosed in U.S. Pat. No. Re. 32,302 does not form until after the fluid, in a single phase, is injected into the subterranean formation. The increased temperature at bottom hole conditions forces a phase change of the liquid carbon dioxide to gaseous carbon dioxide, thereby creating the two-phase character of the fluid.
Foam fracturing processes using nitrogen as a gas phase are known. Because nitrogen is insoluble in water, the unique clean-up advantages obtained by using carbon dioxide are generally not observed in fracturing fluids using only nitrogen. U.S. Pat. No. 4,627,495 by Harris et al., issued Dec. 9, 1986, assigned to Halliburton Company, also discloses a two-phase fracturing fluid. In this instance, the fluid contains between 10% and 96% by volume carbon dioxide or between 10% and 96% by volume nitrogen gas along with various other additives. The fluid has an initial "quality" from about 50% to about 96% or more, wherein "quality" is defined as the percentage of the volume of carbon dioxide (or nitrogen) to the volume of the carbon dioxide (or nitrogen) plus the volume of the aqueous fluid and any other additives at the existing temperature and pressure within the formation. The fluid, like that discussed above, exists in a single phase at wellhead and relies upon increased temperature at bottom hole conditions to form the secondary phase of the fluid. Use of nitrogen is discussed and, when used, the second phase is created at wellhead conditions. However, the patent does not disclose or suggest use of a foam fracturing fluid which incorporates both nitrogen and carbon dioxide.
The improvement in fluid fracturing disclosed in U.S. Pat. No. 4,627,495 is that by controllably reducing the conversion of the carbon dioxide into the gas phase while adding proppant to the fluid, a substantial maintenance of the fluid viscosity is achieved allowing the proppant to be retained in suspension without premature settling of the proppant in the well bore.
U.S. Pat. No. 5,069,283 by Mack, issued Dec. 3, 1991, assigned to The Western Company of North America, discloses use of a fracturing fluid which contains both carbon dioxide and nitrogen gas in the ratio of nitrogen to carbon dioxide in the range of from 0.2:1 to 1:1, with the ratio of the volume of carbon dioxide and nitrogen to the aqueous component of the fluid at wellhead conditions being in the range of about 1:1 to 4:1. Some advantages of the addition of both carbon dioxide and nitrogen are that it provides a two-stage cleanup, it reduces carbonate scale, and helps to stabilize the foam. The patent discloses that, notwithstanding the dissimilar characteristics between nitrogen and carbon dioxide, these two gases can be used together in a fluid fracturing process. The patent discloses that "substantial" quantities of nitrogen are incorporated into the aqueous fracturing fluid, however, the fluids used do not contain greater than equal parts of nitrogen gas to carbon dioxide.
Foam fracturing processes can contribute to deleterious effects upon the environment. For instance, when the foam fracturing fluid is drawn, vented or flared from the subterranean formation, contaminants such as carbon dioxide, carbon monoxide, hydrogen sulfide, propane, methane and well products are released into the atmosphere. Carbon dioxide is a suspected cause of the controversial global warming trend which is commonly known as "the greenhouse effect." In addition, fracturing fluids and their components often remain in the formation after the fracturing process and drilling operations have concluded. In some instances, fluids may cause deterioration of minerals normally present in the formation.
The fracturing fluids disclosed in U.S. Pat. Nos. Re. 32,302, 4,627,495 and 5,069,283 can contribute to deleterious impacts upon the environment. One source of these deleterious impacts is the large concentration of carbon dioxide used in these prior art processes.
Fracturing fluids incorporating carbon dioxide typically require extended periods of time to withdraw, vent or flare the fluid from the formation. This is due to the relatively high solubility of carbon dioxide in water, which requires a substantial amount of time for carbon dioxide to undergo a phase change from a liquid to a gas and completely evacuate from the formation. As a result, during this waiting period a large amount of well product gases and hydrocarbons are released to the atmosphere. This leads to a second source of environmental contamination; the well products themselves.
Carbon dioxide is also very soluble in well products, as much as ten or more times as soluble in well products than in water. Carbon dioxide also has a higher specific gravity than air, which contributes to residual amounts of carbon dioxide remaining in the well after the bulk of the fracturing fluid has been removed. These two factors combine to cause contamination of well products with carbon dioxides which cannot be easily removed from the well products. In many cases, surfactants are added to fracturing fluids containing carbon dioxide in order to reduce the miscibility of the fracturing fluid with the well products. This works only to a limited extent. Another method has been to flare the well products until a majority of the carbon dioxide has been recovered from the formation. This flaring releases combustion products, and other gases from incomplete combustion which are a third source of deleterious environmental impacts from fluid fracturing. Furthermore, when well products are ultimately consumed, impurities in the products reduces the efficiency in which they may be used, and this may represent a fourth source of environmental contamination.
Yet a fifth source of environmental contamination from fracturing processes is the water used in the liquid phase. Water reacts with some minerals present in subterranean formations resulting in the minerals' hydration and deterioration. Deterioration can be severe enough to contaminate well products or reduce the effectiveness of the fracturing job.
It would be desirable to provide a fracturing fluid and method of using said fluid which is capable of performing the fracturing processes described above with reduced deleterious environmental impact. It would be desirable to provide a fracturing fluid and method of using said fluid which vents quicker, maintaining or increasing clean-up efficiency while reducing the amount of carbon dioxide and other contaminant gases which contribute to deleterious environmental impacts to the atmosphere. It would be desirable to provide a fracturing fluid and a method of using said fluid which minimizes the contamination of well products with the fracturing fluid components. It would be desirable to provide a fracturing fluid and method of using said fluid which would minimally deteriorate minerals within the formation.